Hydrocarbon reservoirs are developed with drilling operations using a drill bit associated with a drill string rotated from the surface or using a downhole motor, or both using a downhole motor and also rotating the string from the surface. A bottom hole assembly (BHA) at the end of the drill string may include components such as drill collars, stabilizers, drilling motors and logging tools, and measuring tools. A BHA is also capable of telemetering various drilling and geological parameters to the surface facilities.
Resistance encountered by the drill string in a wellbore during drilling causes significant wear on the drill string, especially the drill bit and the BHA. Understanding how the geometry of the wellbore affects resistance on the drill string and the BHA and managing the dynamic conditions that lead potentially to failure of downhole equipment is important for enhancing efficiency and minimizing costs for drilling wells. Various conditions referred to as drilling dysfunctions that may lead to component failure include excessive torque, shocks, bit bounce, induced vibrations, bit whirl, stick-slip, among others. These conditions must be rapidly detected so that mitigation efforts are undertaken as quickly as possible, since some dysfunctions can quickly lead to tool failures.
One common observed dysfunction includes whirl, which often causes failures in the BHA and damages the drill bit. Whirl refers to a lateral vibration where the rotational axis of the bit does not align with the center of the borehole, and the bit center performs additional rotations around the borehole. Three distinct whirl forms include: (1) backward whirl where the drill string rotates clockwise and the center of the drill string rotates counter-clockwise around the borehole; (2) forward whirl where both drill string and drill-pipe center rotate clockwise but with different rotational speeds; and (3) chaotic whirl where the drill-pipe center does not follow a particular direction but moves in a random and highly unstable fashion.
Tri-axial accelerometers used in the drilling industry measure three orthogonal accelerations related to shock and vibration during drilling operations. The magnitudes of the acceleration data provide a qualitative evaluation of the extent of the drill string vibration. The acceleration data combined with other information may produce a qualitative drilling risk index.
However, prior approaches for quantifying whirl require estimations based on frequency domain computations. This use of the whirl frequency rather than only time domain fails to provide robust results. For example, signal noise may introduce additional peaks in the frequency spectrum and thus limit ability to make accurate determinations of whirl frequency.
Therefore, a need exists for systems and methods to provide reliable determinations of drill string whirl attributes, such as magnitude, orientation and velocity.